Clay formations within a stratigraphic record constitute horizons of low permeability and for that reason sought after targets for different types of subsurface storage systems, such as as host rocks for nuclear waste repositories or as cap rock for reservoirs of injected gas (CO2, H2, CH4).
Mazurek et al. (2009, 2011) present a comprehensive summary of measured and modelled natural tracer profiles across argillaceous formations throughout Europe. Natural tracers included isotopes, dissolved helium and conservative anions such as I-, Br- or Cl-. All of these clay formations were deposited in a marine environment and the initial pore water composition was that of seawater. Changing depositional environments led to the formation of more permeable rocks such as carbonate rocks or sandstones above and below the clay formation. Over geological times, tectonic activity caused uplifting, deformation and erosion. Permeable rocks exposed at the surface became subject to freshwater recharge and turned into freshwater aquifers. These changes in the hydraulic regime and the ensuing change to low-salinity conditions in the permeable rocks initiated diffusive exchange with the saline porewater in the clay formation sandwiched between the aquifers. Current profiles of anions (and other natural tracer) in clay formations therefore are a reflection of the diffusive transport history that began with the activation of the over- and underlying aquifers.
The example of the Opalinus Clay at Mt Terri (Switzerland)
Mt Terri is an anticline in the Folded Jura Mountains in NW Switzerland. The Opalinus Clay at Mt. Terri is a 219 m thick low-permeability sequence deposited in a marine environment during the mid-Jurassic. It is overlain and underlain by permeable carbonate aquifers from the Dogger and the Lias, respectively. Both aquifers exhibit Cl- concentrations that are characteristic of freshwater, indicating the infiltration of surface water. In contrast, the Opalinus Clay owing to its low permeability has maintained its marine signature and exhibits much higher Cl- concentrations, inducing a diffusive flux of Cl- towards the bounding aquifers. Freshwater recharge into these aquifers occurred as a consequence of folding and subsequent erosion of the anticline. The Cl- concentration profile through the Opalinus Clay at Mont Terri shows a distinct maximum which is strongly skewed towards the bottom of the Opalinus Clay. This skew is a result of the fact that recharge of meteoric water occurred first in the Dogger aquifer and later in the Lias aquifer some 6 Ma and 0.5 Ma ago, respectively. Transport is assumed to be by diffusion only. The initial Cl- concentration is 18400 mg/l (i.e. slightly lower than that of seawater) throughout the Opalinus Clay. With these assumptions about the initial and transport conditions and the palaeo-hydrogeology it was possible to reproduce numerically the measured Cl- profile at Mt Terri very well.
